Step 1: Pick two numbers

The first part of this trick requires two numbers. So, pick any two. The trick works a bit better when at least one number is 10 or more. So, for this instructable, I will choose 15 x 19.

Step 2: Dissect the numbers

We can break the equation 19 x 15 down into

19 x (10 + ((1/2) 10))

or

15 x (20 - 1)

Now the first one might seem a little harder, but it's really not. If you have a five in the equation, you can simply multiply a number by 10 (which is simple, just add a 0 at the end), and then take half of it. So 19 x 5 = half of 19 x 10.

That means that 19 x 5 = (1/2) 190

But for now, let's just stick with the second equation because it is simpler.

Step 3: Multiply out

Okay, so we have 15 x (20 - 1)

The distributive property states that we can distribute the 15 to the other numbers, meaning we have (15 x 20) - (15 x 1)

I am sorry for these really short pages, btw.

Step 4: Do some more simple math

15 x 20 is the same as doubling 15 x 10.

Since multiplication has the commutative property, it doesn't matter what order we multiply the numbers in. 15 x 10 x 2 is exactly the same as 10 x 2 x 15, and 2 x 15 x 10, using that rule, we can find that:

2 x (15 x 10) = 2 x 150 = 300

and since we still have to subtract 15 x 1 (which equals 15), we just subtract 15 from 300, giving us 285